login stuff

i am very new to php and right now i am trying to make a members only part to my site. the login part of it doesn't work and i have been trying to diplay a mysql error like this: $result = mysqli_query($connection,$sql)

               or
                         $message= mysql_error();
                         echo "$message<br>";
                         die( );

and of course it doesn't work. please point out what i'm doing wrong. another question is how would i do a password check. like most sites where you login, they have a type password and a retype password box. where would i put this is the form's processor to check the password? i'm sorry that i have to send this to you people, but i need someone more experienced to check this.